The present invention relates to a high strength cold rolled steel sheet having 340 to 440 MPa of tensile strength, which is used for automobile exterior panels such as hoods, fenders, and side panels, and to a method for manufacturing thereof.
Steel sheets used for automobile exterior panels such as hoods, fenders, and side panels have recently often adopted high strength cold rolled steel sheets aiming at improved safety and mileage.
That kind of high strength cold rolled steel sheets are requested to have combined formability characteristics such as further improved deep drawability, punch stretchability, resistance to surface strain (ability of not inducing nonuniform strain on a formed surface) to make the steel sheets respond to the request for reducing the number of parts and for labor saving in press stage through the integration of parts.
To answer the request, recently there have been introduced several kinds of high strength cold rolled steel sheets which use very low carbon steels containing not more than 30 ppm of C as the base material, with the addition of carbide-forming elements such as Ti and Nb, and of solid-solution strengthening elements such as Mn, Si, P. For example, JP-A-112845(1993), (the term xe2x80x9cJP-Axe2x80x9d referred to herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), discloses a steel sheet of very low carbon steel specifying a lower limit of C content and adding positively Mn. JP-A-263184(1993) discloses a steel sheet of very low carbon steel adding a large amount of Mn, further adding Ti or Nb. JP-A-78784(1993) discloses a steel sheet of very low carbon steel with the addition of Ti, further positively adding Mn, and controlling the content of Si and P, thus giving 343 to 490 MPa of tensile strength. JP-A-46289(1998) and JP-A-195080(1993) disclose steel sheets of very low carbon steels adjusting the C content to 30 to 100 ppm, which content is a high level for very low carbon steels, and further adding Ti.
The high strength cold rolled steel sheets prepared from these very low carbon steels, however, fail to have excellent characteristics of combined formability such as deep drawability, punch stretchability, and resistance to surface strain. Thus, these high strength cold rolled steel sheets are not satisfactory as the steel sheets for automobile exterior panels. In particular, these steel sheets are almost impossible to prevent the generation of waving caused from surface strain which interferes the image sharpness after coating on the exterior panels.
Furthermore, to the high strength cold rolled steel sheets used for automobile exterior panels, there have appeared strict requests for, adding to the excellent combined formability, excellent resistance to embrittlement during secondary operation, formability of welded portions corresponding to tailored blank, anti-burring performance under sheering, good surface appearance, uniformity of material in steel coil when the steel sheets are supplied in a form of coil, and other characteristics.
Following is the description of the high strength cold rolled steel sheets according to the present invention, which have excellent characteristics of: combined formability characteristics including deep drawability, punch stretchability, and resistance to surface strain; resistance to embrittlement during secondary operation; formability at welded portions; anti-burring performance; surface characteristics; and uniformity of material in a coil.
Steel sheet 1 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.010% C, 0.05% or less Si, 0.10 to 1.20% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.003% or less 0, 0.01 to 0.20% Nb, by weight; and satisfying the formulae (1), (2), (3), and (4);
xe2x80x83xe2x88x920.46xe2x88x920.83xc3x97log[C]xe2x89xa6(Nbxc3x9712)/(Cxc3x9793)xe2x89xa6xe2x88x920.88xe2x88x921.66xc3x97log[C]xe2x80x83xe2x80x83(1)
10.8xe2x89xa75.49xc3x97log[YP]xe2x88x92rxe2x80x83xe2x80x83(2)
11.0xe2x89xa6r+50.0xc3x97nxe2x80x83xe2x80x83(3)
2.9xe2x89xa6r+5.00xc3x97nxe2x80x83xe2x80x83(4)
where, C and Nb denote the content (% by weight) of C and Nb, respectively, YP denotes the yield strength (MPa), r denotes the r value (average of r values determined at 0, 45, and 90 degrees to the rolling direction), and n denotes the n value (a value in a range of from 1 to 5% strain; average of n values determined at 0, 45, and 90 degrees to the rolling direction).
The Steel sheet 1 is manufactured by the steps of: preparing a continuous casting slab of the steel which has the composition described above; preparing a hot rolled steel sheet by finish rolling the slab at temperatures of Ar3 transformation temperature or more; coiling the hot rolled steel sheet at temperatures not less than 540xc2x0 C.; and cold rolling the coiled hot rolled steel sheet at reduction ratios of from 50 to 85%, followed by continuously annealing thereof at temperatures of from 680 to 880xc2x0 C.
Steel sheet 2 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.14% Nb, by weight, and balance of substantially Fe and inevitable impurities; and having 0.21 or more n value which is calculated from two points of nominal strain, at 1% and 10%, observed in a uniaxial tensile test.
Steel sheet 3 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.15% or less Nb, by weight, and balance of substantially Fe and inevitable impurities; satisfying the formula (6); and having 0.21 or more n value which is calculated from two points of nominal strain, at 1% and 10%, observed in a uniaxial tensile test;
(12/93)xc3x97Nb*/Cxe2x89xa71.2xe2x80x83xe2x80x83(6)
where, Nb*=Nbxe2x88x92(93/14)xc3x97N, and C, N, and Nb denote the content (% by weight) of C, N, and Nb, respectively.
The Steel sheet 3 is manufactured by the steps of: preparing a continuous casting slab of a steel which has the composition described above; preparing a hot rolled steel sheet by finish rolling the slab at temperatures of Ar3 transformation temperature or more; coiling the hot rolled steel sheet at temperatures of from 500 to 700xc2x0 C.; and cold rolling the coiled steel sheet, followed by annealing thereof.
Steel sheet 4 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004t or less N, 0.01 to 0.14% Nb, by weight, and balance of substantially Fe and inevitable impurities; and satisfying the formulae (6) and (7);
(12/93)xc3x97Nb*/Cxe2x89xa71.2xe2x80x83xe2x80x83(6)
TSxe2x88x924050xc3x97Ceqxe2x89xa7xe2x88x920.75xc3x97TS+380xe2x80x83xe2x80x83(7)
where, Ceq=C+(1/50)xc3x97Si+(1/25)xc3x97Mn+(1/2)xc3x97P, TS denotes the tensile strength (MPa), and C, Si, Mn, P, N, and Nb denote the content (% by weight) of C, Si, Mn, P, N, and Nb, respectively.
Steel sheet 5 according to the present invention is a high strength cold rolled steel sheet consisting essentially of: 0.004 to 0.01% C, 0.05% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.03% or less Ti, by weight, and Nb as an amount satisfying the formula (8); 0.03 to 0.1% of a volumetric proportion of NbC; and 70% or more thereof being 10 to 40 nm in size;
1xe2x89xa6(93/12)xc3x97(Nb/C)xe2x89xa62.5xe2x80x83xe2x80x83(8)
where, C and Nb denote the content (% by weight) of C and Nb, respectively.
The Steel sheet 5 is manufactured by the steps of: preparing a continuous casting slab of a steel which has the composition described above; preparing a hot rolled steel sheet by finish rolling the slab at reduction ratios satisfying the formulae (9) through (11); and cold rolling the hot rolled sheet, followed by annealing thereof;
10xe2x89xa6HR1xe2x80x83xe2x80x83(9)
2xe2x89xa6HR2xe2x89xa630xe2x80x83xe2x80x83(10)
HR1+HR2xe2x88x92HR1xc3x97HR2/100xe2x89xa660xe2x80x83xe2x80x83(11)
where, HR1 and HR2 denote the reduction ratio (%) in the finish rolling at the pass just before the final pass and at the final pass, respectively.
Steel sheet 6 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.00100% or less N, 0.036 to 0.14% Nb, by weight; satisfying the formula (12); giving 10 xcexcm or less average grain size and 1.8 or more r value;
1.1 less than (Nbxc3x9712)/(Cxc3x9793) less than 2.5xe2x80x83xe2x80x83(12)
where, C and Nb denote the content (% by weight) of C and Nb, respectively.
The Steel sheet 6 is manufactured by the steps of: preparing a continuous casting slab of a steel which has the composition described above; preparing a sheet bar by either directly rolling the slab or heating the slab to temperatures of from 1100 to 1250xc2x0 C. followed by rough rolling; finish rolling the sheet bar at 10 to 40% of total reduction ratios of the pass just before the final pass and the final pass to produce a hot rolled steel sheet; coiling the hot rolled steel sheet at cooling speeds of 15xc2x0 C./sec or more to temperatures below 700xc2x0 C., followed by coiling at temperatures of from 620 to 670xc2x0 C.; cold rolling the coiled hot rolled steel sheet at 50% or more reduction ratios, followed by heating the steel sheet at 20xc2x0 C./sec or more heating speeds, then annealing the steel sheet at temperatures between 860xc2x0 C. and Ac3 transformation temperature; and temper rolling the annealed steel sheet at 0.4 to 1.0% reduction ratios.
Steel sheet 7 according to the present invention is a high strength cold rolled steel sheet consisting essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S. 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, by weight; and satisfying the formulae (3), (4), (14);
11.0xe2x89xa6r+50.0xc3x97nxe2x80x83xe2x80x83(3)
2.9xe2x89xa6r+5.00xc3x97nxe2x80x83xe2x80x83(4)
1.98xe2x88x9266.3xc3x97Cxe2x89xa6(Nbxc3x9712)/(Cxc3x9793)xe2x89xa63.24xe2x88x9280.0xc3x97Cxe2x80x83xe2x80x83(14)
where, C and Nb denote the content (% by weight) of C and Nb, respectively.
The Steel sheet 7 is manufactured by the steps of: preparing a continuous casting slab of a steel which has the composition described above; preparing a coiled hot rolled steel sheet by finish rolling the slab at 60% or less total reduction ratios of the pass just before the final pass and the final pass; cold rolling the hot rolled steel sheet, followed by annealing thereof.